WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
1. In Baxandall's paper a center tapped treble pot is used which I dispensed with. He also does not include R15 but without it the pentode has no grid reference.
2. I include C8 in the feedback loop to avoid adding another cap (and a potentially troubling time constant). The triode grid gets its ground reference through the tone control circuitry to the pentode grid reference. The problem is that an open bass pot wiper would remove that reference so in the next revision I include a 1 meg resistor (R18) from the input of the tone control circuitry (right end of C8) to ground to cover that possibility. In simulation it seems too have little effect on the response.
3. C10 is included to counteract the continuously rising treble response when trele boost is used. It limits BW to about 40kHz or so.
4. Since it will be driving a length of cable I am considering using a source follower at the output even though the Zin of the power amp will be quite large.
5. Since the triode is biased to accommodate rather large input voltage I am considering incluing the volume control at the output (probably between the pentode and the source follower) to minimize noise and avoid its interfering with the feedback circuit and overall system frequency response.
6. Since the box I plan to house it in has a red LED mounted (power on indicator in previous life) I am considering using it to bias both triodes (left and right channels) as the two together would pass enough current to keep the LED in a fairly linear range and the FB should reduce any residual non-linearity. Just a possibility.